1. Field of the Invention
The invention relates in general to speed pattern generators for motor speed control, and more specifically to speed pattern generators in which a velocity signal is developed in digital form before any analog conversion thereof.
2. Description of the Prior Art
Certain applications of controllable speed motors include a comparison of the desired and actual speeds. The difference or error is used to correct the motor speed to follow the desired or patterned speed. When the motor is used to transport persons, such as an elevator or a train, it is important that the rate of change of acceleration, i.e., jerk, be kept within the limits of comfort, such as a maximum of about 8 ft./sec..sup.3, that the acceleration be limited to a predetermined maximum, such as about 4 ft./sec..sup.2, and that the velocity be limited to the rated or contract speed with little or no overshoot. The attainment of rated speed without overshoot is important, as it enables overspeed detectors and safeties to be set closer to the rated speed without experiencing nuisance trips.
A speed pattern generator for a high speed elevator system, such as an elevator system having a rated speed between 500 fpm. and 1800 fpm., must also be able to handle "short runs", i.e., a run in which the elevator car does not attain rated speed, without exceeding the maximum jerk as the speed pattern changes from maximum positive acceleration to maximum negative acceleration, hereinafter referred to as deceleration. In addition to generating a speed pattern with the safety and comfort of the passengers in mind, it is also important that the speed pattern signal direct the elevator car without undue delay or sluggishness at any part of the speed pattern, in order to provide the most efficient service within the specified jerk, acceleration and velocity constraints.
The more accurate the speed pattern signal, the more efficient the elevator service, as allowances for error degrade response time. Thus, analog computation steps in the development of the speed pattern generator are to be minimized, as analog elements, such as operational amplifiers, are subject to temperature errors and drift.
Certain speed pattern generators of the prior art have taken advantage of digital precision, such as by utilizing registers and adders to perform certain digital calculations. The economic attractiveness and versatility of the microprocessor, which, with its burned-in PROMS forms a dedicated digital computer for performing the necessary logical and computational functions, makes it even more attractive to develop new and improved digital speed pattern generator concepts.
U.S. Pat. No. 3,589,474, entitled "Digital Pattern Generator For Motor Speed Control", which is assigned to the same assignee as the present application, discloses a speed pattern generator in which a digital count is developed from distance pulses generated by car movement. Bias pulses are initially added to the counter to start the car, and the counter counts the incoming distance pulses during acceleration. A non-linear D/A converter develops the speed pattern signals from the count. The count always represents the distance required to stop the car with a deceleration rate equal to the acceleration rate. Upon reaching rated speed, the counter ceases to count the distance pulses, and the speed pattern developed from the constant count is a constant value. When slow-down is initiated, the counter is decremented by the distance pulses, and the speed pattern is derived from this reducing count.
U.S. Pat. No. 3,747,710, entitled "Distance SlowDown Control For Elevator Systems", which is assigned to the same assignee as the present application, discloses generating the slow-down pattern from a digital count responsive to the slow-down distance between the elevator car and the stopping point, as in the hereinbefore mentioned U.S. Pat. No. 3,589,474, with additional means for automatically calibrating the control during each run to enable a smooth transition from the calculated distance-to-go speed pattern to a hatch transducer speed pattern at a point 10 inches from the target floor.
U.S. Pat. No. 4,046,229, entitled "Elevator System", and co-pending application Ser. No. 640,300, filed Dec. 12, 1975, entitled "Elevator System", now U.S. Pat. No. 4,102,436, collectively disclose and claim different aspects of an elevator system in which a first pulse train is generated responsive to actual car movement, and a second pulse train is generated responsive to the desired movement. A counter keeps an accounting of the difference in the counts, and the difference count controls the speed of the elevator car.